Chemistry Reference
In-Depth Information
2500
O
2
•-
2000
1500
HO
2
•
1000
500
0
200
220
240
260
280
300
320
wavelength (nm)
Figure 4.1.
The spectra of
O /HO
2
−
(adapted from Abreu and cabelli [31] with the
2
permission of Elsevier Inc.).
PrOH was estimated to be 1.5 × 10
−5
cm
2
/s [26]. The reduction potentials of
superoxide in water at 1 atm (vs. normal hydrogen electrode [NHE]) vary from
−0.05V (O
2
,
H /HO
•−
) [25]. This suggests the dependence
of reaction kinetics of superoxide on pH.
Spectroscopic methods have been applied to learn fundamental properties
and the kinetics of the reactions of superoxide. Electron spin resonance (ESR)
at ambient temperatures can detect
HO
•
in acidic solution, while the detection
of
O
•−
is only possible at very low temperatures [27, 28]. The spin-trap method
has also been used to detect
HO /O
2
+
•
2
) to −0.33V (
O /O
2
2
• •−
[29, 30]. The most common spectroscopic
method uses characteristic spectra of
HO
•
and
O
•−
(Fig. 4.1) [31]. In the low
UV range, both species have maxima
(
2
ε
225
nm
HO
(
•
)
=
( .
1 40 0 08
±
.
)
×
10
3
/M/s
2
and
(
•−
= ± ×
[16]. The acid-base equilibrium of
superoxide is represented by reaction (4.9). Thus, pH controls the distribution
between
HO
•
and
O
•−
:
ε
245
nm
O
(
)
( .
2 35 0 12
.
)
10
3
/M/s
)
2
HO
•
H O
+
+
•−
p
a
K
=
4 8
.
.
(4.9)
2
2
In recent years, efforts have been made to detect low concentrations
of superoxide. Reactions of
O
•−
with chemical indicators such as tetranitro-
methane, cytochrome
c
, and nitro blue tetrazolium, which form reduced prod-
ucts with intense optical absorbance, have also been utilized to indirectly
quantify superoxide in aqueous solution [32-36]. Progress is also being made
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